This functionality is in beta and is subject to change. The design and code is less mature than official GA features and is being provided as-is with no warranties. Beta features are not subject to the support SLA of official GA features.

The cross-cluster search feature allows any node to act as a federated client across
multiple clusters. In contrast to the tribe node feature, a
cross-cluster search node won’t
join the remote cluster, instead it connects to a remote cluster in a light fashion in order to execute
federated search requests.

Cross-cluster search works by configuring a remote cluster in the cluster state and connecting only to a
limited number of nodes in the remote cluster. Each remote cluster is referenced by a name and a list of seed nodes.
When a remote cluster is registered, its cluster state is retrieved from one of the seed nodes so that up to 3
gateway nodes are selected to be connected to as part of upcoming cross-cluster search requests.
Cross-cluster search requests consist of uni-directional connections from the coordinating node to the previously
selected remote nodes only. It is possible to tag which nodes should be selected through
node attributes (see Cross-cluster search settingsedit).

Each node in a cluster that has remote clusters configured connects to one or more gateway nodes and uses
them to federate search requests to the remote cluster.

Remote clusters can be specified globally using cluster settings
(which can be updated dynamically), or local to individual nodes using the
elasticsearch.yml file.

If a remote cluster is configured via elasticsearch.yml only the nodes with
that configuration will be able to connect to the remote cluster. In other
words, federated search requests will have to be sent specifically to those
nodes. Remote clusters set via the cluster settings API
will be available on every node in the cluster.

This feature was added as Beta in Elasticsearch v5.3 with further improvements made in 5.4 and 5.5. It requires gateway eligible nodes to be on v5.5 onwards.

The elasticsearch.yml config file for a cross-cluster search node just needs to list the
remote clusters that should be connected to, for instance:

Search results are disambiguated the same way as the indices are disambiguated in the request. Even if index names are
identical these indices will be treated as different indices when results are merged. All results retrieved from a
remote index
will be prefixed with their remote cluster name:

The number of nodes to connect to per remote cluster. The default is 3.

search.remote.initial_connect_timeout

The time to wait for remote connections to be established when the node starts. The default is 30s.

search.remote.node.attr

A node attribute to filter out nodes that are eligible as a gateway node in
the remote cluster. For instance a node can have a node attribute
node.attr.gateway: true such that only nodes with this attribute will be
connected to if search.remote.node.attr is set to gateway.

search.remote.connect

By default, any node in the cluster can act as a cross-cluster client and
connect to remote clusters. The search.remote.connect setting can be set
to false (defaults to true) to prevent certain nodes from connecting to
remote clusters. Cross-cluster search requests must be sent to a node that
is allowed to act as a cross-cluster client.